1. Field of the Invention
The present invention relates to the removal of sulfate ions present in aqueous saline solutions, in particular the sulfate ions present in various brine solutions.
2. Description of the Prior Art
Processes for decreasing the content of sulfate ions in electrolysis brines, while avoiding any discharge of toxic elements, have long been a desideratum in this art.
Treatments which generate less effluents, or at least discharging fewer toxic elements such as mercury or barium, are considered especially advantageous.
Thus, the removal of sulfate from brines by precipitation of barium sulfate presents a number of disadvantages:
(i) the requirement to conform to existing regulations relating to the contents of "free" Ba permissible in purification sludges;
(ii) in "mercury loops," the dumping of this contaminated sludge depends on its Hg content; and
(iii) in brine loops of the chlorate processes, the discharges of chromium in BaCrO.sub.4 form are also subject to governmental regulations.
Chemical processes based on the precipitation of sulfate ions do not avoid the discharge of other toxic elements present in the brine, especially mercury, unless a subsequent washing of the sludge is carried out.
In the case of low-concentration brines (100 g/l NaCl) the anionic resin process has a sulfate removal capacity which is very limited by the water balance. It does not, therefore, appear highly advantageous, although it may permit avoiding the discharge of Hg by a rinsing of the resin prior to elution.
Adsorption onto an inorganic substance appears to be the most promising route. Thus, substrates such as zirconium hydroxide exhibit a good affinity for sulfate ions.
The process described in U.S. Pat. No. 3,378,336 comprises purifying a brine on a cationic resin converted into barium form beforehand.
Alkaline earth metals such as magnesium and calcium displace barium which, thus released, precipitates in the form of barium sulfate.
This process presents many disadvantages:
(a) the risk of blocking of the resin by the formation of precipitates,
(b) the release of Ba is a function only of the Ca and Mg content and not of that of sulfate ions. The barium-related problems remain.
U.S. Pat. No. 4,556,463 describes a process for removing sulfate values from brines by two conjugate means:
(1) cascading same over a weakly basic anionic resin and crystallization of the sulfate,
(2) the waters introduced into the anionic loop are employed for diluting the depleted brine to 100 g/l of NaCl.
This latter process has a limited sulfate removal capacity. Indeed, the sulfate removal is directly related to the maximum amount of water that can be introduced into the brine loop.
U.S. Pat. No. 4,415,677 describes a process according to which a cationic resin is impregnated with a zirconium salt. Zirconium oxychloride is next converted into hydroxide form by means of aqueous ammonia:
ZrOCl.sub.2 +2NH.sub.4 OH.fwdarw.ZrO(OH).sub.2 +2NH.sub.4 Cl
Excess aqueous ammonia is washed with the brine. The 26% NaCl solution is acidified to pH=1 to 3 before transport through the column. At T=50.degree. C., pH=1 and 10 B/h (BV: bed volume), the sulfate ion content can be decreased from 970 to 110 ppm.
U.S. Pat. No. 4,405,576 describes the addition of an acrylic acid monomer, which is polymerized in situ, to improve the binding of the zirconium compound. According to the reported example, the load ratio is approximately 5 g of SO.sub.4.sup.= per liter of resin at pH=2.4, T=72.degree. C. and at a sulfate ion concentration of 1.2 g/l.l
U.S. Pat. No. 4,415,678, issued to the same inventive entity, indicates that the load ratio attains 14 g of sulfate ions per liter of resin at pH=1.5 and T=65.degree. C. The elution was conducted with water in all instances. The volumes of the solutions treated in the various reported examples do not exceed 10 BV and no indication is given as to the change in capacity after regeneration.
EP-427,256 to Kanegafuchi describes a process for the extraction of the sulfate ions present in brines using an adsorbent based on zirconium hydroxide/oxide. The ion-exchange mechanisms of the catalyst are:
(i) sorption in acidic medium: 2ZrO(OH).sub.2 +Na.sub.2 SO.sub.4 +2HCl.fwdarw.(ZrOOH).sub.2 SO.sub.4 +2H.sub.2 O+2NaCl
(ii) desorption in basic medium: (ZrOOH).sub.2 SO.sub.4 +2NaOH.fwdarw.2ZrO(OH).sub.2 +Na.sub.2 SO.sub.4
The final product is in the form of particles of a size of from 1 to 20 .mu.m whose effectiveness is proportional to the number of surface OH groups. The sulfate ion content of a brine containing 200 g/l of NaCl can be decreased from 6.2 g/l to less than 0.5 g/l by this adsorbent. However, in the context of sulfate removal from chlorate electrolysis brines, there is a risk that chromium may be bound by the adsorbent and then discharged, the chromate being adsorbed by the adsorbent.
Strelko et al ["The selectivity of the sorption of sulfate ions by hydrated zirconium dioxide", Zhurnal Fizicheskoi Khimii, 64, 408-412 (1990)] describe the selectivity of hydrated zirconium oxide for sulfate adsorption. The zirconium compound employed has a dry solids content of 70%, namely comprising a compound which has an ignition weight loss identical with that of the compound employed in EP-427,256.
Japanese Patents Abstract CA-118127578, relating to a patent of Kanegafuchi, JP-A-300,652/92, describes a process for the removal of sulfate ions from brines, preliminarily dechlorinated, using Zr(OH).sub.4 particles (7-8 .mu.m in size) as ion exchanger and of NaOH as the desorption compound.
Indeed, it is desirable to completely remove the SO.sub.4.sup.= ions present in brines intended for the production of chlorine and soda, on the one hand, and of sodium chlorate, on the other, while avoiding the discharge of toxic materials. None of the aforesaid prior art processes completely avoids these problems.